1. Field of the Invention
The present invention relates generally to an asynchronous Wideband Code Division Multiple Access (WCDMA) communication system, and in particular, to an apparatus and method for minimizing an increase in Peak-to-Average Power Ratio (PAPR) of a transmission signal during data transmission through an Enhanced Uplink Dedicated Transport Channel (EUDCH).
That is, the present invention proposes an optimal Orthogonal Variable Spreading Factor (OVSF) code and In-phase/Quadrature-phase (I/Q) channel allocation apparatus and method for uplink physical channels for EUDCH service.
2. Description of the Related Art
Currently, an uplink of a WCDMA system includes a Dedicated Physical Data Channel (DPDCH) and a Dedicated Physical Control Channel (DPCCH) as typical dedicated physical channels used to transmit user signals. The DPDCH is a data transport channel over which user data such as voice and image data is transmitted, and the DPCCH is a control information transport channel on which DPDCH frame format information and pilot information for DPDCH demodulation and power control are carried.
Recently, technology using a EUDCH, which is an enhanced uplink data-only transport channel, has been proposed to improve a rate and efficiency of packet data transmission in an uplink.
FIG. 1 is a diagram illustrating information exchanged between user equipments and a Node B to perform uplink transmission.
Referring to FIG. 1, UEs 110, 112, 114 and 116 transmit packet data with varying transmission power according to their distances from a Node B 100. The UE 110 which is located the farthest from Node B 100 transmits packet data with the highest transmission power 120 for the uplink channel, while the UE 114 which is located in the nearest to the Node B 100, transmits the packet data with the lowest transmission power 124 for the uplink channel. To improve performance of the mobile communication system, the Node B 100 can perform scheduling such that a level of the transmission power for the uplink channel should be in reverse proportion to the data rate. That is, the Node B 100 allocates the lowest data rate to a UE having the highest transmission power for the uplink channel, and allocates the highest data rate to a UE having the lowest transmission power for the uplink channel.
FIG. 2 is a diagram illustrating information exchanged between a UE and a Node B to perform uplink transmission. That is, FIG. 2 illustrates a basic procedure required between a Node B 200 and a UE 202 for packet data transmission through an EUDCH.
Referring to FIG. 2, in step 210 involving a process of transmitting/receiving messages through a dedicated transport channel, a EUDCH is set up between the Node B 200 and the UE 202. In step 212, the UE 202 transmits information regarding a desired data rate and information estimating an uplink channel condition to the Node B 200. The information estimating an uplink channel condition includes transmission power of an uplink channel transmitted by the UE 202 and a transmission power margin of the UE 202.
Node B 200, which receives the uplink channel transmission power, can estimate a downlink channel condition by comparing the uplink channel transmission power with reception power. That is, Node B 200 considers that an uplink channel condition is fair if a difference between the uplink channel transmission power and the uplink channel reception power is small, and considers that the uplink channel condition is poor if the difference between the transmission power and the reception power is high. When the UE 202 transmits transmission power margin to estimate an uplink channel condition, Node B 200 can estimate the uplink transmission power by subtracting the transmission power margin from the known possible maximum transmission power for the UE 202. Node B 200 determines the maximum data rate for an uplink packet channel of the UE 202 using the estimated channel condition of the UE 202 and information on a data rate required by the UE 202.
The maximum data rate is notified to the UE 202 in step 214. The UE 202 determines a data rate for transmission packet data within a range of the notified possible maximum data rate, and in step 216, transmits the packet data to the Node B 200 at the determined data rate.
Herein, uplink physical channels supporting the EUDCH service include a Dedicated Physical Data CHannel (DPDCH), a Dedicated Physical Control Channel (DPCCH), a High-Speed Dedicated Physical Control CHannel (HS-DPCCH) for High-Speed Downlink Packet Access (HSDPA) service, an Enhanced Dedicated Physical Data Channel (E-DPDCH) for the EUDCH service, and an Enhanced Dedicated Physical Control CHannel (E-DPCCH) for the EUDCH service.
That is, in step 216, the UE 202 transmits an E-DPCCH to provide frame format and channel coding information of the E-DPDCH channel, and transmits packet data through the E-DPDCH. Herein, the E-DPCCH can also be used for transmission of an uplink data rate required by the UE 202 and transmission power margin, and transmission of pilot information required by the Node B 200 for demodulation of the E-DPDCH.
If the UE 202 transmits separate physical channels in addition to the existing physical channels in order to transmit EUDCH packet data as described above, the number of physical channels transmitted in the uplink increases, causing an increase in a Peak-to-Average Power Ratio (PAPR) of an uplink transmission signal. Generally, the PAPR increases as the number of simultaneously transmitted physical channels increases.
Since the increase in the PAPR may increase distortion of transmission signals and an allowed Adjacent Channel Leakage power Ratio (ACLR), a Radio Frequency (RF) power amplifier in a UE requires power back-off which reduces amplifier's input power to prevent the foregoing problem. If the UE performs power back-off, the result is a reduction in reception power at a receiver in a Node B, causing an increase in error rate of the received data or a reduction in cell coverage.
Accordingly, in order to prevent the increase in PAPR, the UE intends to transmit the EUDCH over the existing physical channel such as a DPDCH on a time division basis, instead of transmitting the EUDCH on a separate physical channel. However, the process of transmitting the EUDCH over the existing physical channel on a time division basis increases implementation complexity.
As a result, a WCDMA system has proposed a method for multiplying the physical channels by Orthogonal Vector Spreading Factor (OVSF) codes satisfying mutual orthogonality before transmission in the uplink. The physical channels multiplied by the OVSF codes can be distinguished in a Node B.
FIG. 3 is a diagram illustrating a tree structure for OVSF codes commonly used in a WCDMA system.
Referring to FIG. 3, the OVSF codes can be simply generated in a calculation process shown in Equation (1) to Equation (3).
                              C                      ch            ,            1            ,            0                          =        1.                            Equation        ⁢                                  ⁢                  (          1          )                                                  [                                                                      C                                      ch                    ,                    2                    ,                    0                                                                                                                        C                                      ch                    ,                    2                    ,                    1                                                                                ]                =                              [                                                                                C                                          ch                      ,                      1                      ,                      0                                                                                                            C                                          ch                      ,                      1                      ,                      0                                                                                                                                        C                                          ch                      ,                      1                      ,                      0                                                                                                            -                                          C                                              ch                        ,                        1                        ,                        0                                                                                                                  ]                    =                                    [                                                                    1                                                        1                                                                                        1                                                                              -                      1                                                                                  ]                        .                                              Equation        ⁢                                  ⁢                  (          2          )                                                  [                                                                      C                                      ch                    ,                                          2                                              (                                                  n                          +                          1                                                )                                                              ,                    0                                                                                                                        C                                      ch                    ,                                          2                                              (                                                  n                          +                          1                                                )                                                              ,                    1                                                                                                                        C                                      ch                    ,                                          2                                              (                                                  n                          +                          1                                                )                                                              ,                    2                                                                                                                        C                                      ch                    ,                                          2                                              (                                                  n                          -                          1                                                )                                                              ,                    3                                                                                                      ⋮                                                                                      C                                      ch                    ,                                          2                                              (                                                  n                          +                          1                                                )                                                              ,                                                                  2                                                  (                                                      n                            -                            1                                                    )                                                                    -                      2                                                                                                                                            C                                      ch                    ,                                          2                                              (                                                  n                          +                          1                                                )                                                              ,                                                                  2                                                  (                                                      n                            +                            1                                                    )                                                                    -                      1                                                                                                    ]                =                              [                                                                                C                                          ch                      ,                                              2                        n                                            ,                      0                                                                                                            C                                          ch                      ,                                              2                        n                                            ,                      0                                                                                                                                        C                                          ch                      ,                                              2                        n                                            ,                      0                                                                                                            -                                          C                                              ch                        ,                                                  2                          n                                                ,                        0                                                                                                                                                              C                                          ch                      ,                                              2                        n                                            ,                      1                                                                                                            C                                          ch                      ,                                              2                        n                                            ,                      1                                                                                                                                        C                                          ch                      ,                                              2                        n                                            ,                      1                                                                                                            -                                          C                                              ch                        ,                                                  2                          n                                                ,                        1                                                                                                                                          ⋮                                                  ⋮                                                                                                  C                                          ch                      ,                                              2                        n                                            ,                                                                        2                          n                                                -                        1                                                                                                                                  C                                          ch                      ,                                              2                        n                                            ,                                                                        2                          n                                                -                        1                                                                                                                                                              C                                          ch                      ,                                              2                        n                                            ,                                                                        2                          n                                                -                        1                                                                                                                                  -                                          C                                              ch                        ,                                                  2                          n                                                ,                                                                              2                            n                                                    -                          1                                                                                                                                          ]                    .                                    Equation        ⁢                                  ⁢                  (          3          )                    
As illustrated in FIG. 3, orthogonality is secured between the OVSF codes having the same spreading factor (SF). In addition, for two codes having different SF values, if a code having a larger SF value cannot be generated from a code having a lower SF value using Equation (3), orthogonality is acquired between the two codes.
A description thereof will be made by way of the following example.
For SF=4, Cch,4,0=(1,1,1,1) is orthogonal with Cch,2,1=(1,−1) but is not orthogonal with Cch,2,0=(1,1).
As another example, comparing SF=256 OVSF codes with the Cch,2,1=(1,1), since OVSF codes with SF=0˜127 are generated from the Cch,2,1=(1,1), orthogonality is not secured therebetween. That is, as a higher data rate is required, an OVSF code with a lower SF value is used, and when a plurality of physical channels are simultaneously transmitted, the OVSF codes should be allocated such that orthogonality is secured therebetween.
Although two physical channels use the same OVSF code, if they are separately transmitted through an I channel and a Q channel of a transmitter, a receiver can separate the two physical channel signals without mutual interference and demodulate the separated physical channel signals, because the signals transmitted on the I channel and the Q channel are carried by carriers having a 90°-phase difference.
As described above, an increase in uplink PAPR depends on the number of physical channels simultaneously transmitted in the uplink, a power ratio between physical channels, an OVSF code used for each physical channel, and I/Q channel allocation for each physical channel.
In the WCDMA system to which the EUDCH technology is applied, if E-DPCCH and E-DPDCH channels for transmission of EUDCH packet data are simultaneously transmitted in addition to the uplink channels, the PAPR increases undesirably.